1. Field of the Invention
Metering of free-flowing media or gases, and the use thereof.
2. Description of the Background
(Meth)acrylic acid and (meth)acrylic esters are important products in the chemical industry, which serve as starting materials for many important products. A maximum yield and a particularly high purity coupled with low preparation costs are therefore essential for the economic success of a preparation process for such an important product. Even relatively small improvements with regard to the yields, the service lives of the plants or similar process features lead to a significant advance with regard to the amount of undesired by-products and the preparation costs.
The methacrylamide used to prepare methacrylic acid can preferably be obtained by what is known as the ACH process. Proceeding from hydrogen cyanide and acetone, acetone cyanohydrin is prepared in a first step, and is then converted to methacrylamide. These steps are described, inter alia, in U.S. Pat. No. 7,253,307, EP-A-1 666 451 and PCT/EP2007 059092.
Acetone cyanohydrin is prepared by commonly known processes (see, for example, Ullmanns Enzyklopädie der technischen Chemie, 4th Edition, Volume 7). Frequently, the reactants used are acetone and hydrogen cyanide. The reaction is an exothermic reaction. In order to counteract decomposition of the acetone cyanohydrin formed in this reaction, the heat of reaction is typically removed by a suitable apparatus. The reaction can in principle be conducted as a batchwise process or as a continuous process; when a continuous method is preferred, the reaction is frequently performed in a loop reactor configured correspondingly.
The acetone cyanohydrin prepared by different known preparation processes is typically subjected to a distillative workup. This involves freeing the stabilized crude acetone cyanohydrin of low-boiling constituents by means of an appropriate column. A suitable distillation process can, for example, be conducted using only one column. However, it is likewise possible in the case of a corresponding purification of crude acetone cyanohydrin to use a combination of two or more distillation columns, also in combination with a falling-film evaporator. It is additionally possible to combine two or more falling-film evaporators or else two or more distillation columns with one another.
The crude acetone cyanohydrin is generally transferred from the storage to the distillation with a temperature of about 0 to about 15° C., for example a temperature of about 5 to about 10° C. In principle, the crude acetone cyanohydrin can be introduced directly into the column. However, it has been found to be useful in some cases when the crude cool acetone cyanohydrin first absorbs, by means of a heat exchanger, some of the heat from the product already purified by distillation. Therefore, in a further embodiment of the process described here, the crude acetone cyanohydrin is heated by means of a heat exchanger to a temperature of about 60 to 80° C.
The distillative purification of the acetone cyanohydrin is effected by means of a distillation column having more than 5 and preferably more than 10 trays, or by means of a cascade of two or more correspondingly suitable distillation columns. The column bottom is preferably heated with steam. It has been found to be advantageous when the bottom temperature does not exceed a temperature of 140° C.; it has been possible to achieve good yields and good purification when the bottom temperature is not greater than about 130° C. or not higher than about 110° C. The temperature figures are based on the wall temperature of the column bottom.
The crude acetone cyanohydrin is supplied to the column body in the upper third of the column. The distillation is preferably performed under reduced pressure, for example at a pressure of about 50 to about 900 mbar, especially of about 50 to about 250 mbar, and with good results between 50 and about 150 mbar.
At the top of the column, gaseous impurities, especially acetone and hydrogen cyanide, are withdrawn, and the gaseous substances removed are cooled by means of a heat exchanger or a cascade of two or more heat exchangers. In this context, preference is given to using brine cooling with a temperature of about 0 to about 10° C. This gives the gaseous constituents of the vapours the opportunity to condense. The first condensation stage can take place, for example, at standard pressure. It is, however, likewise possible, and has in some cases been found to be advantageous, when this first condensation stage is effected under reduced pressure, preferably at the pressure which exists in the distillation. The condensate is passed on into a cooled collecting vessel and collected there at a temperature of about 0 to about 15° C., especially at about 5 to about 10° C.
The gaseous compounds which do not condense in the first condensation step are removed from the reduced pressure space by means of a vacuum pump. It is possible in principle to use any vacuum pump. However, it has been found to be advantageous in many cases when a vacuum pump which, owing to its design, does not lead to the introduction of liquid impurities into the gas stream is used. Preference is therefore given here to using, for example, dry-running vacuum pumps.
The gas stream which escapes on the pressure side of the pump is conducted through a further heat exchanger which is preferably cooled with brine at a temperature of about 0 to about 15° C. This condenses constituents which are likewise collected in the collecting vessel which collects the condensates already obtained under vacuum conditions. The condensation performed on the pressure side of the vacuum pump can be effected, for example, by means of one heat exchanger, but also with a cascade of two or more heat exchangers arranged in series in parallel. Gaseous substances remaining after this condensation step are removed and sent to any further utilization, for example a thermal utilization.
The condensates collected can likewise be utilized further in any way. However, it has been found to be extremely advantageous from an economic point of view to recycle the condensates into the reaction for preparation of acetone cyanohydrin. This is preferably done at one or more points which enable access to the loop reactor. The condensates may in principle have any composition provided that they do not disrupt the preparation of the acetone cyanohydrin. In many cases, the predominant amount of the condensate will, however, consist of acetone and hydrogen cyanide, for example in a molar ratio of about 2:1 to about 1:2, frequently in a ratio of about 1:1.
The acetone cyanohydrin obtained from the bottom of the distillation column is first cooled in a first heat exchanger by the cold crude acetone cyanohydrin supplied to a temperature of about 40 to about 80° C. Subsequently, the acetone cyanohydrin is cooled by means of at least one further heat exchanger to a temperature of about 30 to about 35° C. and optionally stored intermediately.
In a further process element, acetone cyanohydrin is subjected to a hydrolysis. At various temperature levels, and after a series of reactions, this forms methacrylamide as the product.
The conversion is accomplished in a manner known per se to the person skilled in the art by reaction between concentrated sulphuric acid and acetone cyanohydrin. The reaction is exothermic, and so heat of reaction can be removed from the system in an advantageous manner.
Here too, the conversion can again be performed in a batchwise process or in continuous processes. The latter has been found to be advantageous in many cases. When the reaction is performed in the course of a continuous process, the use of loop reactors has been found to be useful. Loop reactors are known in the technical field. These may be configured, for example, in the form of tubular reactors with recycling. The reaction can be effected, for example, in only one loop reactor. However, it may be advantageous when the reaction is performed in a cascade of two or more loop reactors.
A suitable loop reactor in the context of the process described has one or more feed sites for acetone cyanohydrin, one or more feed sites for concentrated sulphuric acid, one or more gas separators, one or more heat exchangers and one or more mixers. The loop reactor may comprise further constituents, such as conveying means, pumps, control elements, etc.
As already described, the hydrolysis of acetone cyanohydrin with sulphuric acid is exothermic. In parallel to the main reaction, several side reactions take place, which lead to lowering of the yield. In the preferred temperature range, the decomposition of acetone cyanohydrin, likewise an exothermic and rapid reaction, plays a significant role. The heat of reaction which arises in the course of the reaction, however, has to be at least substantially removed from the system, since the yield falls with increasing operating temperature and rising residence time. It is possible in principle to achieve rapid and comprehensive removal of the heat of reaction with corresponding heat exchangers. However, it may also be disadvantageous to cool the mixture too greatly before the metered addition of acetone cyanohydrin, since high turbulence is needed both for mixing and for efficient heat removal. Since the viscosity of the mixture being stirred rises significantly with falling temperature, the flow turbulence falls correspondingly, in some cases down to the laminar range, which leads in the heat exchanger to less efficient heat removal, and to slower and less homogeneous mixing when the acetone cyanohydrin is metered in.
What is required is rapid mixing of acetone cyanohydrin and reaction mixture, since the acetone cyanohydrin should react before it decomposes owing to the heating. Fine dropletization of the reactant, which means a large specific interface area, causes a preference for the desired reaction at the droplet surface over the heating of the droplet volume with subsequent decomposition. A fine distribution of acetone cyanohydrin has been found to be advantageous, since the reaction takes place at the droplet surface.
Furthermore, excessively low temperatures in the reaction mixture can lead to crystallization of constituents of the reaction mixture on the heat exchangers. This further worsens the heat transfer, which causes a significant decline in yield. Furthermore, the loop reactor cannot be charged with the optimal amounts of reactants, and so the efficiency of the process suffers overall.